The growth of cable distribution networks has caused television manufacturers to provide television receivers with "cable-ready" or "cable-compatible" tuning systems. Cable distribution networks provide RF signals having carriers with non-standard frequencies which may be offset from respective standard broadcast frequencies. Accordingly, "cable-compatible" tuning systems must be capable of tuning such non-standard frequency RF signals. It is desirable that a "cable-compatible" tuning system be capable of tuning "pay" channels for which the video information is scrambled, usually by modifying a video synchronization component.
Tuning systems for non-standard frequency RF signals utilize searches in which the local oscillator frequency is changed in a range around the nominal local frequency for the selected channel and either one or both of an automatic fine tuning and a video synchronization signal is examined to determine when to terminate the search. One such system is disclosed in U.S. Pat. No. 4,405,947, entitled "Dual Search Mode Type Tuning System" issued in the names of J. Tults and M. P. French in September 1983.
If the non-standard frequency tuning provisions evaluate a synchronization component, scrambled channels may not be tunable. If only the AFT signal is utilized, while scrambled channels can be tuned, erroneous signals, such as sound carriers can be tuned.